1. Field of the Invention
The present invention relates to an image forming apparatus using an electrophotographic process, particularly to a color-image forming apparatus for forming different color images by using a plurality of laser beams and a method for correcting a color-misregistration in the image forming apparatus.
2. Related Background Art
In the case of a conventional laser printer using an electrophotographic system, a laser beam modulated in accordance with an image signal is reflected by a scanner having a polygon mirror (rotational polygon mirror) and caused to scan the surface of a photosensitive member to form an image. The photosensitive member often used has a drum-shape and is generally referred to as a photosensitive drum. At the time of applying the system to a color laser printer, a color image is formed on a sheet-like recording medium by superimposing a plurality of images of different colors (e.g. four colors of yellow (Y), magenta (M), cyan (C), and black (BK)). Following configurations are available for achieving the superimposition technique.
As one of the configurations, a latent image is formed by scanning the surface of a photosensitive drum with a first color-image signal, a developer is attached to the latent image in order to visualize the latent image, the latent image is transferred to recording paper, then the photosensitive drum is cleaned, a latent image is formed again by scanning the surface of the same photosensitive drum with a second color-image signal, and the above development and transferring steps are carried out. The developer of the second color-image signal is a developer for a second color. Then, the above steps are repeated for third and fourth color-image signals in the same manner. Thus, one image is formed by superimposing a plurality of images developed on the same recording paper.
Moreover, in the case of another configuration, one image is recorded by using photosensitive drums in the number equal to a plurality of color-image signals, forming respective latent images on the photosensitive drums corresponding to the color-image signals one to one, visualizing and developing the latent images with different-color developers, and successively transferring the developed images to recording paper. In this case, one laser, one scanner, one BD (Beam Detect) sensor for detecting the image write timing of the laser, and one photosensitive drum are commonly prepared for one image signal. Therefore, when there are a plurality of image signal to be superimposed, it is necessary to use lasers, scanners and photosensitive drums of the same number as that of the image signals.
In the case of the former configuration, an electrophotographic process including a series of charging, exposure, development, transfer, and cleaning is applied to a first color-image signal and then, the same process is applied to a second color-image signal again, and moreover, the same process must be applied to a third color-image signal and a fourth color-image signal in time series. Thus, the print time for one sheet becomes prolonged.
The latter configuration has an advantage that printing can be performed in a short time compared to the former configuration. However, it is necessary to prepare lasers, scanners, and photosensitive drums of the same number as that of color-image signals used and therefore, an apparatus increases in size and becomes expensive.
Moreover, because images of respective colors are superimposed in either of the above configurations, the so-called color misregistration due to mismatch between color image positions tends to occur. Particularly, in the case of the latter configuration, the registration of color images is hardly matched because each color image is formed by using a different scanner and a different photosensitive drum. Therefore, the registration matching is performed for every color. For example, means is used which performs correction by forming a registration-detecting pattern image on an intermediate transferring belt (ITB) or electrostatic transportation belt (ETB), reading the image by a registration-detecting sensor, and feeding back the image to the write-start position of the image.
The registration-detecting sensor irradiates the registration-detecting image pattern formed on the ITB or ETB with a light emitted from a light source, and focuses and reads the reflected light by a light-receiving sensor. Then, a temporal intensity change of a signal from the light-receiving sensor when the registration-detecting pattern passes is detected as positional-misregistration information and electrical processing is applied to the information.
Usually, the rotational speed of the scanner is raised in order to shorten the printing time of laser printer. The conventional scanner rotational speed of a laser printer is normally set to a high-speed rotation of 20,000 rpm or higher. Moreover, the scanner uses a polygon mirror. When there is an error of a deflection angle, a positional fluctuation occurs on a photosensitive drum depending on the optical path length of a laser beam. Therefore, as for the scanner, it is necessary that the inclination error of each face of the polygon mirror is very small and the vibration of the polygon mirror due to high-seed rotation is little.
Therefore, to obtain stable high-speed rotation of the polygon mirror, a large motor is necessary and it is also necessary to limit the inclination error of each face of the polygon mirror. Thus, an accurate processing technique is required for a scanner fabrication process. Therefore, the fabrication yield is low and the cost is greatly increased.
Moreover, the above apparatus provided with a plurality of scanners inadvantageously becomes large-scaled and expensive. Therefore, to cut down the cost, the following are proposed: an apparatus using a common scanner for a plurality of colors (Japanese Patent Publication No. 4-51829) and an apparatus using a common scanner and providing a BD sensor only for one light source among a plurality of light sources (Japanese Patent Laid-Open No. 4-313776). In the case of the apparatus disclosed in Japanese Patent Laid-Open No. 4-313776, a plurality of light sources are constituted so that they are simultaneously allowed to scan the photosensitive member by the aid of different faces of a polygon mirror and light sources other than the light source provided with a BD sensor can be estimated by a BD signal of the light source provided with the BD sensor because the rotational phase difference (angle difference) of the polygon mirror is previously known.
As described above, in the case of a color-image forming apparatus normally provided with a plurality of polygon mirrors to form a color image by scanning different photosensitive drums for each of colors, PLL control is performed so that the numbers of revolutions of the scanner motors are made equal. Moreover, to accurately adjust the conveyance-directional write-start position of each color, rotation is controlled while keeping the plane phase of each polygon mirror in a specific positional relation.
In the case of a conventional image-forming apparatus, however, it is impossible to correct the write-start position of a misregistration amount equal to or less than one line in the conveyance direction between a plurality of colors to be scanned by one polygon mirror. Therefore, to correct the misregistration amount equal to or less than one line in the conveyance direction between a plurality of colors to be scanned by different polygon mirrors, if the misregistration amount equal to or less than one line between a plurality of colors to be scanned by the above one polygon mirror is large, the above conveyance-directional color misregistration amount between a plurality of colors to be scanned by different polygon mirrors is superimposed and thereby, the color misregistration amount between colors (four colors) may be increased.